1. Field of the Invention
The present invention relates to a position input apparatus to be used for example, on an operation panel of a copying machine and to a position detector used for the position input apparatus.
2. Description of the Prior Art
A position input apparatus on a plane surface may be needed, for example, to specify the position of a copy image when editing work such as trimming, masking and moving of a copy image done in a copying machine.
Conventionally, a position input apparatus is used includes position detecting means mode from a strip of conductor and a strip of resistor disposed opposite each other. There is conventionally a complementary operating means for performing complementary operation of a pressed position in accordance with an output from the position detecting means when it is pressed.
In the above position detecting means, when a portion of the resistor body is pressed while a given voltage is being applied across the resistor electrode, a voltage corresponding to the pressed position is developed across the resistor electrode.
On the other hand, the voltage values that the position detecting means outputs when the reference points provided at both ends of the position detecting means are pressed are those previously stored in the complementary operating means. A pressed position can be obtained by performing complementary operation of the pressed position on the basis of the voltage that the position detecting means outputs when it is pressed and the previously stored voltage outputs from the reference points.
However, because of the effects of variations in the resistivity and cross section of the resistor, the relationship of the output voltage relative, to the pressed position does not always result in a linear valve in the position detecting means, as shown for example by the curve L in FIG. 1. For that reason, the voltages that the position detecting means outputs when the two reference points X1 and X2 provided in the position detecting means are pressed are V1 and V2 respectively, pressing the position X0, for example, causes the voltage V0' to be output instead of the voltage V0. If the difference between these voltage V0, and V0 is greater than the resolution of the complementary operating means, the pressed position is erroneously recognized.
Accordingly, the above, prior art position input apparatus has a problem which is that the accuracy of position input cannot be improved even if resolution of the complementary operating means is enhanced, and therefore it tends to erroneously recognize the position input. In many cases, such a tablet type position input apparatus have generally high output impedance. In particular, when the electrode is not pressed, they stay in entirely a high impedance state. This tends to cause noise in the signals output from the tablet type position input apparatus because of the effects such as electromagnetic wave and static electricity. Accordingly, such tablet type position input apparatus is a problem in that an apparatus provided with such tablet type position input apparatus tend to malfunction or fail to operate.
According to other prior art, a tablet type position input apparatus is used which, for example, which includes a planar resistor larger than a document to be copied and a stylus pen having a pointed tip with a built-in switch. In this table type position input apparatus, a position on a plane surface can be input by pressing the desired position on the planar resistor with the stylus pen. However, the tablet type position input apparatus of the above prior art has a problem which is that a stylus pen must be used each time when inputting a position, and therefore, the operatability is not very good. Operatability may be improved if the resistor plate is pressed directly with a finger. In this case, however, the area pressed with a finger is substantially larger than when the resistor is pressed with a stylus pen with a pointed tip, and a delicate positional difference results each time the resistor is pressed even if the same area is pressed, leading to a worsened reproducibility and a substantial drop in the accuracy of position input.